According to many psychologists, hypnotherapists and personal achievement experts, the most effective way to write an affirmation is in the present tense (for example, "I am 9% body fat," or "I have a 30 inch waist).
One reason for that is because your subconscious - the part of your mind that generates autonomic bodily functions and automatic behaviors (habits) - is very peculiar in the way it can take things literally.
If your affirmation is, "I will lose 20 pounds", your subconscious mind says, "WHEN?" It's possible that your subconscious will interpret that statement as, "Someday in the future I will lose 20 pounds." If you keep saying an affirmation in future tense, the literal interpretation by the subconscious mind is to keep the goal in the future, delaying action and postponing achievement.
The subconscious is influenced in many ways - not just words. What is equally, if not more important as the written affirmation, is the mental picture, the emotion and the belief underlying your words.
Words, mental pictures, emotions and belief could collectively be called INTENT and it's your INTENT that counts because words mean different things to different people. In some cultures, words do not even exist for certain things or concepts, and if words do not exist, then the experience or emotion of that word cannot take place.
INTENT also has to do with belief. A wise teacher once said that INTENT is what you want AND what you believe, combined.
It's possible you might want something, and write it down as a goal or say it as an affirmation, but on a deeper level, you might not really believe you can get it. So "wanting," to some people, is negative because they have wanted things and not received them and that's because they didn't believe they could have them.
If you intend to achieve something, this means you must want something and you must believe you will get it and you deserve it, so regardless of whether your affirmation or goal is phrased in present tense or not, your subconscious mind gets the message loud and clear.
Suppose this is your goal: "I will complete the Burn The Fat body transformation challenge in the best shape of my life, weighing 172 ripped pounds with 7% body fat on May 31st."
As you read that goal, if you are creating a mental picture of yourself winning and you attach a lot of emotion to the image, steadily holding the picture with belief and faith (you truly INTEND to make it happen), you will be motivated from within to take the actions necessary and it will become reality - even though you said, "I will" (future tense) instead of "I am" (present tense).
There's more to the subconscious goal achievement mechanism than just words - there is the mental picture, emotion, belief and many other factors that also influence the subconscious mind and help change your behavior, which helps you achieve your goals. - Tom Venuto